1. Field of the Invention
The present invention relates to a mobile telecommunication system supporting an enhanced uplink service. More particularly, the present invention relates to a power setting method and a power setting apparatus for transmitting data based on characteristics of uplink channels.
2. Description of the Related Art
An Enhanced-uplink Dedicated Channel (hereinafter referred to as ‘E-DCH’) has been proposed to improve the performance of uplink packet transmission in a Wideband Code Division Multiple Access (hereinafter referred to as ‘WCDMA’) system. Along with the introduction of the E-DCH, a plan is under discussion to use Adaptive Modulation and Coding (hereinafter referred to as ‘AMC’), Hybrid Automatic Retransmission Request (hereinafter referred to as ‘HARQ’) and Node B control scheduling methods for an uplink.
FIG. 1 is a basic conceptual view illustrating a situation where the E-DCH is used.
Referring to FIG. 1, a Node B 100 supports the E-DCH and detects channel states of User Equipments (hereinafter referred to as ‘UE’) 101, 102, 103, 104 using the E-DCH to perform scheduling suitable for the respective UEs via paths 111, 112, 113 and 114, respectively. That is, the Node B 100 assigns a low data rate to the UE 104 at a remote location and assigns a high data rate to the UE 101 at a close location while maintaining a noise rise value below a threshold noise rise value.
FIG. 2 illustrates basic transmission/reception procedures of the E-DCH.
Referring to FIG. 2, in step 203, a Node B 200 and a UE 202 establish the E-DCH. The establishment of the E-DCH includes a procedure of delivering messages through a dedicated transport channel.
In step 204, the UE 202 informs the Node B 200 of scheduling information. The scheduling information may be information on the UE's transmit power, from which uplink channel information can be derived, or include information on extra power, with which the UE can transmit data, and the amount of data to be transmitted, which is stored in a buffer of the UE.
In step 211, the Node B 200 performs scheduling for several UEs including the UE 202 while monitoring the scheduling information received from the respective UEs.
When the Node B 201 determines to permit uplink packet transmission to the UE 202, it transmits scheduling assignment information to the UE 202 in step 205. Here, the scheduling assignment information may include an allowed rate, allowed timing, KEEP/UP/DOWN for an uplink and the like.
In step 212, the UE 202 determines a Transport Format (hereinafter referred to as ‘TF’) of the E-DCH to be transmitted over the uplink using the scheduling assignment information.
In steps 206 and 207, the UE 202 transmits a Transport Format Resource Indicator (TFRI), information related to the determined TF, and uplink packet data (UL packet data) including E-DCH data to the Node B 201.
In step 213, the Node B 200 determines whether the information relating to the TF and the E-DCH data have errors. At this time, the Node B determines the information to be a Negative Acknowledge (hereinafter referred to as ‘NACK’) if there is any error in the information and determines the information to be a Positive Acknowledge (hereinafter referred to as ‘ACK’) if there is no error in the information.
In step 208, the Node B transmits ACK/NACK information to the UE 202 through an ACK/NACK channel according to the result judged in step 213. At this time, the UE 202 transmits new data if it receives the ACK and retransmits the previous data if it receives the NACK.
FIG. 3 illustrates an example of data transmission through the E-DCH in the WCDMA system.
As shown in FIG. 3, if data 316 to be transmitted, including video telephony, uploading of multimedia mail, games, etc., occurs, a UE 317 spreads the data 316 using a code assigned to a physical channel 318 and then transmits it to a Node B 319.
The E-DCH is mapped to and transmitted through an Enhanced Dedicated Physical Data Channel (hereinafter referred to as ‘E-DPDCH’) using a code multiplexing scheme. The E-DPDCH may coexist with a DPDCH to which a DCH, a typical uplink transmission channel of a UE, is mapped, a Dedicated Physical Control Channel (hereinafter referred to as ‘DPCCH’) which carries control information related to the DPDCH, and an Enhanced DPCCH (hereinafter referred to as ‘E-DPCCH’) which carries control information related to the E-DPDCH. A UE sets transmit powers of the respective physical channels according to the current maximum transmit power which is allowed to the UE. The maximum transmit power may be determined by the transmission capability of a power amplifier of the UE and a minimum value of a transmit power set by a network. At this time, the transmit powers of the physical channels other than the DPCCH are determined according to power ratios relative to the DPCCH.
FIG. 4 is a block diagram illustrating a structure of a conventional physical layer transmission stage of a UE.
Referring to FIG. 4, the UE 317 encodes data to be transmitted through a data channel via a coding block 305. Control information necessary for the reception of the data channel is also generated separately for a control channel. Here, the data channel represents the DPDCH or the E-DPDCH and the control channel represents the DPCCH or the E-DPCCH.
The control information and the encoded data are modulated by modulators 300, 306, respectively. The modulated control information and data are spread by spreaders 301, 307 using channelization codes of the data channel and the control channel, that is, Cc and Cd, respectively and then are transmitted to gain scalers 302, 308, respectively. The spread control information and data are multiplied by gain factors given to the control channel and the data channel, that is, βc, βd in the gain scalers 302, 308, respectively and then are multiplexed by a multiplexer 303. The multiplexed data is input to a scrambler 304 to be scrambled by a scrambling code Sdpch,n of the DPCH and then is converted into a Radio Frequency (RF) signal through an RF unit 309 to be wirelessly transmitted by an antenna.
The gain factor is a value for setting powers of the relevant physical channel based on the DPCCH which is subjected to power control and is set according to data sizes and services types of the respective physical channels. The gain factor, one of the components constituting a TF, is set according to a Transport Format Combination (hereinafter referred to as ‘TFC’). The gain factor is determined by a TFC selection part of an upper layer, which in turn determines a format of transmission channel data, and is transmitted to the physical layer. The physical channel sets the transmit power of each physical channel according to the gain factor. At this time, the UE scales the gain factor of each physical channel such that it does not exceed a maximum allowed power.
If only a TFC satisfying an allowable power level is selected through the TFC selection in a case of desiring to further use the E-DCH in addition to the transmission channels defined in the conventional system, the gain factors of all the physical channels are equally scaled when total transmit power exceeds the maximum allowed power. The E-DCH supports a HARQ technique in which demodulation at a reception stage is possible only by always using the same transmission format as that of the initial transmission even during retransmission. Thus, the TFC selection part always selects the same TFC as that of the initial transmission irrespective of allowable TFCs when E-DCH data is retransmitted. In general, the transmit power during retransmission is set to the same level as that of the initial transmission.
In some situations, there may be a case where DCH data does not exist at the initial transmission of the E-DCH and the DCH data occurs at retransmission. There may also be a situation where the E-DCH data must be retransmitted under the condition of a fixed TF and fixed transmit power of the DCH when a Transmission Time Interval (hereinafter referred to as ‘TTI’) of the E-DCH is set to less than the minimum TTI of the DCH. In such situations, if a UE uses the same power as that of the initial transmission for retransmission of the E-DCH data, it is likely to result in total transmit power exceeding the maximum allowed power. Although it is possible to transmit all the physical channels within the maximum allowed power by equally scaling down powers of all the physical channels while maintaining their power ratios, transmission qualities of the respective physical channels may not be ensured.
FIG. 5 is a diagram illustrating an example of problems occurring during conventional retransmission of E-DCH data.
As shown in the drawing, a UE initially transmits E-DCH data through an E-DPDCH in time slot T1. Since there is no DCH data in time slot T1, total transmit power 401 does not exceed the maximum allowed power (Pmax) 407. However, in time slot T2 in which retransmission of the E-DCH data occurs, DCH data is transmitted through a DPDCH and thus the total transmit power 402 including those of the E-DPDCH and the DPDCH exceeds the maximum allowed power (Pmax) 407. Thus, as designated by reference numeral 405, the transmit powers of the E-DPDCH, the DPDCH and a DPCCH are equally scaled down. As a result of this, after the transmit powers of the physical channels have been scaled down, the total transmit power 404 does not exceed the maximum allowed power 407 in time slot T3.
However, qualities of all the E-DPDCH, the DPDCH and the DPCCH at a reception stage are all lowered in time slot T3 because such physical channels are transmitted using less power than those in time slot T2. In particular, if the transmit powers of all the physical channels are always equally scaled in a case where the DCH and the E-DCH have different priorities from each other, transmission quality of the DCH or the E-DCH may deteriorate due to retransmission. To give an example, even if the DCH is used for a voice call having higher priority, DCH data may be transmitted with a very low level of power in some time slots within one TTI due to retransmission of the E-DCH data having lower priority, which results in quality deterioration of the voice call.
Therefore, in a case where an E-DCH supporting HARQ exists, there is a strong desire to provide a technique for efficiently controlling the transmit power of each physical channel when the total transmit power of a UE exceeds the maximum allowed transmit power.